Electrodes used in electric arc furnaces typically comprise joined sections of graphitic or non-graphitic carbon to form an electrode column. As the electrode column is consumed during operation of the furnace, new electrode sections are periodically joined at the non-consuming end of the electrode column. The sections are typically joined by a joint comPrising a female threaded socket in one section which joins a threaded male nipple in the adjoining section. The male nipple is usually formed by screwing a double ended threaded nipple into a threaded socket. These joints are generally satisfactory but are subject to unscrewing which results in loose joints, breakage of the electrode at the joint, and loss of the electrode section below the joint.
One solution to this Problem is provided by the so-called Pitch-reservoir-filled joint (PRF), as described in U.S. Pat. No. 2,735,704, by providing pitch reservoirs in the electrode joint, such as by lateral Perforations in the nipple of the joint. During operation of the arc furnace, the heat of resistance generated by the electric current melts the pitch, which then flows between the mating threads of the nipple and the socket. Further heating of the joints causes the pitch to coke, thereby bonding the mating threads together. While the resulting coke bond inhibits unscrewing, vibration and thermal stresses on the joint often break the coke bond between the threads of the joint. Furthermore, the bonding coke is not evenly distributed between the mating threads, resulting in concentration of thermal and mechanical stresses at the bonded surfaces. This can lead to breakage of the joint, particularly at weak points where the joint has been perforated for the pitch reservoirs. For example, in joints with pitch reservoirs in lateral holes through the joint nipple, breakage of the nipple at the holes is a problem. It would be desirable, therefore, to provide a method which is more effective than the pitch reservoir method to prevent unscrewing while avoiding weakening of the joint.
An other problem with PRF joints occurs when there is a breakage of the nipple, leaving a broken half of the nipple in the socket of the electrode segment above the broken joint. In order to reuse the electrode segment, the broken portion of the nipple remaining in the socket must be removed to allow attachment of an unbroken nipple. With PRF joints, coke bonds the threads of the nipple and the socket together, rendering it impossible or very difficult to unscrew and remove the broken nipple portion without damaging the threads of the socket. Thus, in order to reuse the electrode segment, the end of the electrode segment containing the broken nipple is generally cut off, and a new threaded socket machined into the end of the segment.